Why in transformer frequency does not change?

 In transformer frequency does not change but the voltage is changed

The Frequency is dependant on speed and poles of the generator. The sinosoidal wave in primary winding of a transformer induces voltage and frequency in real time. The ratio of No. of  turns in primary and secondary winding increase or decrease the output voltage so the voltage is variable and frequency is constant..

Change in frequency cause increas in excitation by the way increase heat in transformer and need protection

What keeps the frequency the same is the sin wave time. which doesnt change in both side of the transformer.

current induced in secondary coil is dependent on primary coil of transformer. For every high pulse in primary coil current is induced in secondary, therefore frequency remains same.